Dark ‘ring’ inside shotgun barrel (?)

[Bergie]

I was shooting a few clays yesterday, got home and was cleaning the guns and saw something I never really noticed before- there is a dark shadow or ring inside the barrel of my 11-87 that isn’t in either my old 870 Wingmaster nor my newer 870 Express. It seems to be a few inches down the tube from the chamber. (If this were a rifle, that’s a no-brainer, but a shotgun?) What do I not know?

[smurfman]

Looks like the forcing cone. The forcing cone is the part of the barrel where inside dimensions go from chamber diameter to bore diameter. The longer the forcing cone, the easier it is to see. Under roughly 1.5" the forcing cones can be difficult to see but over that length they get easier to see.

If this were the other end of the barrel one would think it to be the choke.

[Rip Van Winkle]

Agree, it's the forcing cone. Some are shorter and more noticeable than others.